Vega-Lite creation with AI chat

**What the feature is (as Title)**
Users can create Vega-Lite charts by describing what they want in plain language through chat. Vega is a powerful charting library supporting a wide range of visualizations (https://vega.github.io/vega/).

**Value proposition**
Vega charts have traditionally required learning a complex specification language. With this feature, users can skip that learning curve entirely and generate chart types not available in Lens — such as Sankey diagrams, flow diagrams, scatter plots, bubble charts, small multiples, radar charts, sunbursts, and more — using a simple prompt.

**Expected outcome**
- A prompt like "Create a flow diagram to see how my API requests flow across services" produces a new chart directly on the dashboard.
- Users can edit existing Vega charts through chat without needing to understand or write Vega syntax.

**Optional: Key user stories / use cases**
https://github.com/elastic/kibana/issues/259708

Start by reading the linked Kibana issue 259708 and the Vega documentation URL to understand the proposed chart-generation flow. The issue names no files, tests, or entry points; done would mean defining and implementing chat-based creation and editing of Vega-Lite charts in the dashboard.

Written by the indexing model from the issue text.